JP2019059963A - Manufacturing method of steel sheet having low yield ratio - Google Patents

Abstract

To provide a manufacturing method of a steel sheet having a low yield ratio.SOLUTION: The manufacturing method of a steel sheet having a low yield ratio comprises: a hot rolling step of finish rolling a steel slab having a predetermined composition at a finish rolling temperature of 880°C or higher and coiling at 550°C or higher and 670°C or lower to form a hot rolled sheet; a cold rolling step of cold rolling the hot rolled sheet with a cold rolling reduction of 65% or more and 92% or less to form a cold rolled sheet; and an annealing step of heating the cold rolled sheet and annealing at an annealing temperature T in a temperature range of 760°C or higher and 950°C or lower. The relationship between a thickness true strain εt in the cold rolling step and a heating rate HR(°C/s) from 650°C to the annealing temperature T in the annealing step satisfies a following equation(2). -14.0≤ln(HR)-5(εt)≤-8.0(2). However, note that εt in the equation(2) is calculated by the following equation(3). εt=-ln(1-CR/100)(3). However, note that CR in the equation(3) is a cold-rolling reduction(%) in the cold-rolling step.SELECTED DRAWING: None

Description

  The present invention relates to a method of manufacturing a steel sheet having a low yield ratio. The present invention is particularly useful for applications such as automobile outer panel, etc., manufacture of a cold rolled steel sheet having a tensile strength (TS) of not less than 270 MPa and less than 500 MPa and a low yield strength and less generation of wrinkles during press molding. It relates to the method.

In recent years, in order to regulate the amount of CO ₂ emission from the viewpoint of global environmental protection, improvement of automobile fuel consumption is required. In addition, in order to ensure the safety of the occupant in the event of a collision, there is also a demand for improved safety centered on the collision characteristics of the car body. Thus, weight reduction of the car body and reinforcement of the car body are actively promoted.

  In order to satisfy the weight reduction and reinforcement of the car body simultaneously, it is said that the weight reduction by increasing the strength of the component material and reducing the thickness is effective, as long as there is no problem with rigidity, and recently high tension Steel plates are actively used in auto parts.

  The weight reduction effect increases as the strength of the steel plate used increases, so in the automobile industry, for example, there is a trend to use a steel plate with a tensile strength (TS) of 390 MPa or more as a panel material for inner and outer plates.

  On the other hand, many automobile parts made of steel plates are formed by press processing, and therefore, steel plates for automobiles are required to have excellent press formability. However, high-strength steel plates deteriorate formability compared to ordinary mild steel plates, and so there are many problems in promoting weight reduction of automobiles. In panel materials for exterior panels, it is important to strictly control the surface quality after pressing, for example, to eliminate local irregularities called surface strain around the handle of the door.

  For such surface strain, it is necessary to lower the yield strength, and an attempt is made to lower the yield ratio YR [= (YS / TS) × 100] which is the ratio of the yield strength (YS) to the tensile strength (TS). It is done.

  For example, Patent Document 1 discloses a steel plate excellent in stretch flangeability and excellent in press formability that is a low YS characteristic and a method of manufacturing the same. In patent document 1, C: 0.0040 to 0.015 wt%, Nb: 0.04 to 0.25 wt%, Nb / (7.75 × C) = 1.5 to 2.5, and carbon with Nb By fixing sufficiently and heating in the annealing stage above the recrystallization temperature, by controlling the tension with 0.3 × Nb / (7.75 × C) −0.25 ≦ tension T ≦ 2.0, A low density region of NbC is formed in the vicinity of grain boundaries to achieve low YS.

  Further, the steel plate disclosed in Patent Document 2 has a structure in which a second phase consisting mainly of martensite is dispersed in a suitable amount in a ferrite structure, and YP is reduced compared to a solid solution-strengthened steel such as a conventional IF steel. Ru. Moreover, the said steel plate has high BH and is excellent also in dent resistance. However, when this steel plate is press-formed in parts such as doors, the amount of generation of surface strain is larger than that of a conventional 340 MPa grade bake-hardened steel plate, and thus further reduction in YP is required.

In Patent Document 3, a hot-rolled sheet having a predetermined component is cold-rolled and annealed in a temperature range of Ac _{1 to} Ac ₃ points, and then at a cooling rate of 3 to 20 ° C./s at a temperature of 550 to 750 ° C. A method of producing a steel sheet is disclosed, in which primary cooling is performed to a temperature range of 1. and secondary cooling to 200 ° C. or less at a cooling rate of 100 ° C./s or more. However, since the method described in Patent Document 3 requires rapid cooling after annealing, it can be applied to a continuous annealing line not subjected to plating treatment, but a zinc plating bath maintained at 450 to 500 ° C. during cooling after annealing. In principle, it is difficult to apply in a continuous hot dip galvanizing line which is subjected to plating treatment by immersion in

JP 2001-131681 A Japanese Patent Publication No. 62-40405 JP, 2006-233294, A

An object of the present invention is to provide a method for producing a steel sheet having a low yield ratio.
The steel plate having a low yield ratio in the present invention means a steel plate having a yield ratio (YR) of 50% or less.

  Conventionally, in order to improve deep drawability, the atomic ratio of Nb to C (12/93) × (Nb / C) [Nb and C each represents the content (mass%) of each element] of 1.0 Although it is sufficiently higher than that of NbC, it is possible to lower the yield strength (YS) by controlling the dispersion and to increase the tensile strength (TS) by precipitation strengthening when the amount of NbC precipitates is large, but the yield ratio is high. It was difficult to make (YR) 50% or less. Therefore, as a result of further investigation, in order to obtain a steel plate having a sufficiently low yield ratio, the atomic ratio of Nb and C is set to a limited range of 0.8 or more and 1.1 or less, and It has been found that it is important to control the cold rolling rate and the heating rate in the annealing process.

The present invention has been made based on such findings and has the following configurations.
[1] mass%,
C: 0.0040 to 0.0120%,
Si: 0.70% or less,
Mn: 0.50 to 1.80%,
P: 0.005 to 0.05%,
S: 0.01% or less,
Al: 0.005 to 0.3%,
N: 0.005% or less,
Nb: 0.025 to 0.110%
Contains and
In a steel slab having a composition in which the atomic ratio of Nb to C is 0.80 or more and 1.10 or less and the balance is Fe and an unavoidable impurity represented by the following formula (1):
Finish rolling The finish rolling is performed at a temperature of 880 ° C. or higher, and wound at 550 ° C. or more and 670 ° C. or less to form a hot rolled sheet;
A cold rolling step of subjecting the hot-rolled sheet to cold rolling at a cold rolling ratio of 65% to 92% to obtain a cold-rolled sheet;
And annealing the cold-rolled sheet at an annealing temperature T in a temperature range of 760 ° C. or more and 950 ° C. or less.
A steel sheet having a low yield ratio, wherein the relationship between the thickness true strain εt in the cold rolling process and the heating rate HR (° C./s) from 650 ° C. to the annealing temperature T in the annealing process satisfies the following equation (2) Manufacturing method.
(12/93) x (Nb / C) (1)
However, Nb and C in said Formula (1) represent content (mass%) of each element.
−14.0 ≦ ln (HR) −5 (εt) ≦ −8.0 (2)
However, εt in the above equation (2) is calculated by the following equation (3).
εt = −ln (1-CR / 100) (3)
However, CR in the said (3) Formula is a cold-rolling rate (%) in a cold-rolling process.
[2] In addition to the above-mentioned composition, further, the steel slab is, by mass%,
B: The manufacturing method of the steel plate which has a low yield ratio as described in [1] which contains 0.0010% or less.
[3] The method for producing a steel sheet having a low yield ratio according to [1] or [2], wherein the steel sheet surface is further plated after the annealing step.

ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the steel plate which has a low yield ratio can be provided.
The steel plate produced by the production method of the present invention has an unprecedented low yield ratio. Moreover, since the addition of alloy elements such as Mo and Cr is limited, the plating property and the surface appearance are good. Therefore, it can be suitably used as a steel plate for automobile outer plates.

  In the present invention, when the C content is in the range of C: 0.0040 to 0.0120%, the atomic ratio of Nb to C (12/93) × (Nb / C) is 0.80 ≦ (12/93). ) Fix carbon at a lower level than the conventional IF steel with x (Nb / C) 1. 1.10, and control the cold rolling rate in the cold rolling process and the heating rate in the annealing process It has been found that ferrite single phase steel can achieve an unprecedented low yield ratio. Although the reason for this is not necessarily clear, it is considered as follows.

  In the past, from the viewpoint of deep drawability, Ti and Nb were added in excess of the equivalent of carbon in order to reduce the solid solution carbon to the limit, but the carbides inhibit this and the yield strength of the ferrite structure is lowered sufficiently. It was not done. Therefore, by adding Nb in the equal amount or slightly lower range of carbon, it was possible to make the carbide easy to yield. In this case, since the precipitation state of carbides in hot rolling is unstable, a part of the carbide (NbC) is dissolved in cold rolling, and the carbides are reprecipitated in the heating stage (the annealing stage). It has become clear by such evaluations. The dissolution of carbides depends on the strain energy of cold rolling, that is, the cold rolling rate, and the dispersion of carbides upon reprecipitation influences the heating rate in the annealing process, so the two are optimized. It is considered that even a single ferrite phase could achieve an unprecedented low yield ratio.

  The present invention will be described in detail below. Hereinafter, the content of the element is indicated by mass% unless otherwise specified.

  First, the reason for limiting the component composition of the steel slab of the present invention will be described.

C: 0.0040 to 0.0120%
C is an important element in the present invention together with Nb described later. The upper limit is made 0.0120%, considering that excessive addition is not preferable in order to obtain good deep drawability of automobile skin panels. C is preferably as low as possible from the viewpoint of deep drawability, but from the viewpoint of utilizing precipitation control of NbC, addition of 0.0040% or more is necessary. The C content is preferably 0.0050% or more. The C content is preferably 0.0110% or less.

Si: 0.70% or less Si has the effect of enabling solid solution strengthening without significantly reducing deep drawability. In order to acquire the said effect, it is preferable to contain Si 0.20% or more, More preferably, it contains 0.35% or more. On the other hand, if Si is contained in excess of 0.70%, a red scale is generated during hot rolling, which deteriorates the surface appearance of the steel plate. In addition, since the wettability of the plating is deteriorated when the hot dip galvanizing is performed to cause generation of uneven plating and the plating quality is deteriorated, the Si content is set to 0.70% or less. The Si content is preferably 0.50% or less.

Mn: 0.50 to 1.80%
Mn is effective not only for strengthening but also for preventing hot cracking due to S. From such a viewpoint, Mn needs to be contained at 0.50% or more. Mn is contained preferably at least 0.55%, more preferably at least 1.00%. On the other hand, the excessive addition thereof degrades deep drawability and weldability, so the Mn content is limited to 1.80%. The Mn content is preferably 1.40% or less.

P: 0.005 to 0.05%
P has the effect of solid solution strengthening. However, if it is less than 0.005%, not only the effect does not appear but also the dephosphorization cost is increased in the steel making process. Therefore, P is contained at 0.005% or more. On the other hand, excessive addition exceeding 0.05% causes P to segregate at grain boundaries and degrades resistance to secondary processing brittleness and weldability. Therefore, the upper limit of the content of P is 0.05%. The P content is preferably 0.010% or more. The P content is preferably 0.03% or less.

S: 0.01% or less S is an impurity and is a cause of hot cracking, and is present as inclusions in steel to degrade various properties of the steel sheet, so it is preferable to reduce as much as possible, but 0.01 Since it is acceptable up to%, the S content is made 0.01% or less. The S content is preferably 0.005% or less.

Al: 0.005 to 0.3%
In addition to being useful as a deoxidizing element of steel, Al also has the effect of fixing solid solution N to improve the normal temperature aging resistance, and therefore it should be contained 0.005% or more. On the other hand, addition exceeding 0.3% causes cost increase due to high alloying, and further causes surface defects, so the Al content is made 0.3% or less. The Al content is preferably 0.015% or more. Moreover, Al content is preferably 0.15% or less.

N: 0.005% or less When N is too large, the normal temperature aging resistance is deteriorated and a large amount of Al or Ti is required to be added. Therefore, it is preferable to reduce as much as possible, and the upper limit is made 0.005%. The N content is preferably 0.0030% or less.

Nb: 0.025 to 0.110%
Nb is an element contributing to the deep drawability, having the function of refining the hot rolled sheet structure and depositing and fixing C as NbC in the hot rolled sheet. From such a viewpoint, Nb is contained at 0.025% or more. On the other hand, the addition of excessive Nb increases the cost and increases the hot rolling load. When the amount of NbC is large, the yield strength is high, so the Nb content is 0.110% or less.

In the present invention, it is important to control the atomic ratio of Nb to C in the present invention. When the atomic ratio of Nb and C shown in the following formula (1) is less than 0.80, a large amount of solid solution carbon is present at the hot rolling stage, and the deep drawability after cold rolling annealing decreases. Further, the presence of solid solution carbon in the cold rolled annealed material is not preferable because it raises the yield strength. Therefore, the atomic ratio of Nb and C needs to be 0.80 or more and 1.10 or less. The atomic ratio of Nb to C is preferably 0.85 or more. The atomic ratio of Nb to C is preferably 1.05 or less.
(12/93) x (Nb / C) (1)
However, Nb and C in said Formula (1) represent content (mass%) of each element.

  The above is the basic component of the steel slab of the present invention. In the present invention, the balance other than the above-described components is a composition of iron and unavoidable impurities.

  The composition of the steel slab of the present invention can further contain B: 0.0010% or less in addition to the above components.

B: 0.0010% or less B is an element having an effect of improving the secondary processing brittleness of steel, and can be contained as necessary. However, since the effect is saturated when the content exceeds 0.0010%, the B content is preferably 0.0010% or less. The B content is more preferably 0.0008% or less.

  In addition, as said unavoidable impurity, V, Mo, and Cu can be added in the range which is 0.1% or less in total in the range which does not affect the target characteristics.

  The steel plate of the present invention includes a so-called plated steel plate which has been subjected to surface treatment such as electroplating or hot-dip plating. Plating refers to pure zinc, an element that forms an alloy, zinc-based alloy plating to which an alloy element is mainly added with zinc, or Al-based alloy plating to which an alloy element is mainly added with Al or Al, Mg, It also includes a plating layer conventionally applied to the surface of a steel sheet, such as Ni or an alloy containing these elements.

  The component composition of the steel plate in the present invention is the same as the composition of the above-described steel slab.

  Next, the method of manufacturing the steel plate of the present invention will be described. In the following description, the temperature is a surface temperature of a steel slab, a hot-rolled sheet or the like unless otherwise specified.

  In the present invention, in the hot rolling step, the steel slab having the above composition is subjected to finish rolling at a finish rolling outlet temperature of 880 ° C. or higher.

  The steel slab used in the production method of the present invention is preferably produced by a continuous casting method in order to prevent macrosegregation of the components, but may be produced by an agglomerating method or a thin slab casting method. Also, after the steel slab is manufactured, it is cooled to room temperature and then reheated, and then added to the conventional method without heating, as it is warm, directly loaded into a heating furnace and hot rolled, or slight heat retention It is also possible to apply an energy saving process such as direct feed rolling or direct rolling in which hot rolling is immediately performed after the

  It is desirable that the steel slab heating temperature be as low as possible to improve the deep drawability by developing {111} recrystallization texture by coarsening precipitates. However, if the heating temperature is less than 1000 ° C., the rolling load increases and the risk of trouble occurrence during hot rolling increases, so it is preferable to set the steel slab heating temperature to 1000 ° C. or more. In addition, it is preferable to set the upper limit of the steel slab heating temperature to 1300 ° C., because the scale loss increases with the increase of the oxidation weight.

  The steel slab heated under the above conditions is subjected to hot rolling for rough rolling and finish rolling. Here, the steel slab is made into a sheet bar by rough rolling. The conditions for rough rolling do not need to be particularly defined, and may be determined according to a conventional method. Further, it is needless to say that it is an effective method to utilize a so-called sheet bar heater which heats a sheet bar from the viewpoint of lowering the steel slab heating temperature and preventing troubles during hot rolling.

  Next, the sheet bar is finish-rolled to form a hot-rolled sheet. In the present invention, the finish rolling outlet temperature (FT) is 880 ° C. or higher. This is to obtain a fine hot-rolled sheet structure which can obtain excellent deep drawability after cold rolling and recrystallization annealing. When the FT is less than 880 ° C., the structure has a worked structure and not only does not develop a {111} texture after cold rolling annealing, but also the rolling load at the time of hot rolling becomes high. Therefore, FT is 880 ° C. or higher. On the other hand, when FT exceeds 980 ° C., the structure becomes coarse, which may also hinder the formation and development of {111} recrystallization texture after cold rolling annealing, and there is a possibility that deep drawability can not be secured. Therefore, it is preferable to set FT to 980 ° C. or less.

  Moreover, in order to reduce the rolling load at the time of hot rolling, lubricating rolling may be performed between some or all of the passes of finish rolling. Lubricated rolling is also effective from the viewpoint of the uniformity of the steel sheet shape and the homogenization of the material. The coefficient of friction in lubricated rolling is preferably in the range of 0.10 to 0.25. Furthermore, it is also preferable to join continuous sheet bars and to set it as the continuous rolling process which carries out finish rolling continuously. Applying a continuous rolling process is also desirable from the viewpoint of the operation stability of hot rolling.

Winding temperature (CT): 550 ° C. or more and 670 ° C. or less The coil winding temperature is 550 ° C. or more and 670 ° C. or less. While this temperature range is a temperature range suitable for precipitating NbC in the hot-rolled sheet, particularly when the CT exceeds the above upper limit, the crystal grains become coarse and the strength is lowered, and the deep drawability after cold rolling annealing is It will interfere. It is necessary to partially dissolve NbC produced in the hot rolling stage by cold rolling and control NbC so as to lower the yield strength after annealing. Therefore, the upper limit of the winding temperature is set to 670 ° C.

  After the hot rolling step, the hot rolled sheet is subjected to cold rolling to perform a cold rolling step to form a cold rolled sheet. The hot rolled sheet may be pickled after the hot rolling step and before the cold rolling step. Pickling may be performed under normal conditions.

Cold rolling reduction (CR): 65% or more and 92% or less The cold rolling step is an important step for controlling the distribution of NbC in order to obtain a low yield ratio. In the cold rolling step, it is necessary to divide NbC precipitated in the hot rolling step by cold working to dissolve fine unstable NbC, and CR is at least 65% or more. Moreover, high cold rolling reduction is generally effective also from the viewpoint of deep drawability, and if the reduction ratio (cold rolling reduction) is less than 65%, the {111} recrystallization texture does not develop and excellent deep drawability is obtained. I can not get it. On the other hand, when CR exceeds 92%, not only the effect is saturated, but also the load on the roll during cold rolling is increased, so the upper limit is made 92%.

Annealing temperature: Annealing temperature T in a temperature range of 760 ° C. or more and 950 ° C. or less
Next, the cold rolled sheet subjected to the cold rolling process is heated, and an annealing process is performed to perform annealing at an annealing temperature T in a temperature range of 760 ° C. or more and 950 ° C. or less. In this annealing step, at least recrystallization needs to be performed. For this reason, annealing at a temperature range of 760 ° C. or higher is at least necessary. On the other hand, when the temperature is higher than 950 ° C., the recrystallized grains are significantly coarsened and the characteristics are significantly degraded. For this reason, the annealing in the annealing step is performed at an annealing temperature T in a temperature range of 760 ° C. or more and 950 ° C. or less. The annealing temperature T is preferably 800 ° C. or more. The annealing temperature T is preferably 900 ° C. or less.

The relationship between the thickness true strain εt in the cold rolling process and the heating rate HR (° C./s) from 650 ° C. to the annealing temperature T in the annealing process satisfies the following formula (2).
−14.0 ≦ ln (HR) −5 (εt) ≦ −8.0 (2)
However, εt in the above equation (2) is calculated by the following equation (3).
εt = −ln (1-CR / 100) (3)
However, CR in the said (3) Formula is a cold-rolling rate (%) in a cold-rolling process.

  In order to obtain a low yield ratio, it is necessary to control the distribution of NbC in the final annealing stage. In this case, it is necessary to control the formation of NbC and the growth state at the stage of heating in the annealing step after the division and disappearance by the energy of cold working, and the cold rolling in the cold rolling step It is necessary to control both the rate and the heating rate in the annealing process. The heating rate in the annealing step has almost no effect on controlling the heating rate below 650 ° C considering diffusion of carbon and Nb, and the heating rate from 650 ° C to the annealing temperature (annealing temperature T) is important .

  As a result of investigation by the inventor, the relationship between the thickness true strain εt in the cold rolling process and the heating rate HR (° C./s) from 650 ° C. to the annealing temperature T in the annealing process is the above (2) It was found that the yield ratio of the steel sheet after cold rolling annealing can be made very low by satisfying the formula. When the heating rate in the annealing step is slow, the effect of energy in cold working becomes insensitive, but when the heating rate in the annealing step is fast, it is necessary to further increase the working rate in the cold rolling step.

  The heating rate HR (° C./s) from 650 ° C. to the annealing temperature T in the annealing step is not particularly limited as long as the above equation (2) is satisfied, but from the point of production efficiency etc. / S or more is preferable. In addition, in view of passage stability and the like, 30 ° C./s or less is preferable.

  The cooling rate after the above-mentioned annealing step is not particularly limited, but it is preferable to cool from the annealing temperature T to 300 ° C. at an average cooling rate of 5 ° C./s or more. It is preferable that the average cooling rate be 5 ° C./s or more from the temperature T to the overaging treatment temperature.

  In addition, after the annealing step, plating treatment such as electroplating treatment or hot-dip plating treatment may be performed to form a plating layer on the surface of the steel plate.

  For example, when performing hot dip galvanizing treatment which is often used for automobile steel plates as plating treatment, the above annealing process is performed in a continuous hot dip plating line, and it is immersed in a hot dip galvanization bath following cooling after the annealing process. A hot-dip galvanized layer may be formed on the surface, or an alloying treatment may be further performed to produce an alloyed hot-dip galvanized steel sheet. In that case, it is preferable to cool so that the average cooling rate up to 300 ° C. is 5 ° C./s or more, even after cooling out after the hot-dip plating pot or after alloying treatment.

  The cooling after the annealing step may be performed on an annealing line, and once cooled to room temperature, hot-dip plating may be performed on a hot-dip plating line, or an alloying treatment may be further performed.

  Here, it goes without saying that the plating layer is not limited to pure zinc and zinc-based alloy plating, but may be various plating layers conventionally applied on the surface of a steel plate, such as Al or Al-based alloy plating.

  Further, temper rolling or leveler processing may be applied to the above-mentioned steel plates (cold-rolled annealed plate and plated steel plate) for the purpose of shape correction, adjustment of surface roughness and the like. The elongation percentage of temper rolling or leveler processing is preferably within the range of less than 0.5% in total. When the elongation rate is 0.5% or more, the yield strength is increased, and surface strain at the time of pressing tends to occur.

Next, examples of the present invention will be described.
A steel slab having the composition shown in Table 1 is heated to 1250 ° C. and roughly rolled to form a sheet bar, and then a hot rolling process is performed by a hot rolling step of performing finish rolling under the conditions shown in Table 2. After pickling these hot rolled sheets, a cold rolling process was performed at a cold rolling ratio shown in Table 2 to obtain cold rolled sheets. Subsequently, these cold rolled sheets were subjected to continuous annealing under the conditions shown in Table 2 on a continuous annealing line. Furthermore, temper rolling having an elongation of 0.2% was applied to the obtained steel sheet (cold-rolled and annealed sheet).

  The tensile properties and r value of the obtained cold rolled annealed sheet were measured. The measuring method is as follows.

(1) Tensile properties From each cold-rolled annealed sheet obtained, a JIS No. 5 tensile test specimen is taken in the direction of 90 ° (C direction) with respect to the rolling direction, and the crosshead speed is 10 mm / in accordance with JIS Z 2241. A tensile test was performed at min to determine yield stress (YS) and tensile strength (TS). Further, the yield ratio (YR) was determined from (YS, TS) by (YS / TS) × 100.

(2) r value measurement JIS5 tensile test from the rolling direction (L direction), 45 ° direction (D direction) to the rolling direction, and 90 ° direction (C direction) to the rolling direction of each cold rolled annealed sheet obtained A piece was taken. Determine the width strain and thickness strain of each test piece when applying 10% uniaxial tensile strain to these test pieces, and determine the average r value (average plastic strain ratio) according to the provisions of JIS Z 2254 , This was taken as r value. In addition, it can be evaluated that it is excellent in deep drawability, so that this r value is large.

  As is clear from Table 2, in each of the inventive examples, a steel sheet having a low yield ratio of 50% or less for YR was obtained. Further, in the examples of the present invention, steel plates having an average r value of 1.3 or more were obtained. On the other hand, in the comparative example manufactured on the conditions which remove | deviate from the range of this invention, the steel plate whose YR is low enough was not obtained. In addition, when structure | tissue observation was performed about the steel plate of the example of this invention, all the steel plates of this invention had a ferrite single phase structure.

  According to the present invention, it is possible to inexpensively and stably manufacture a cold rolled steel sheet having a high r value with an average r value of 1.3 or more at YR 50% or less, and the industrially remarkable effect is exhibited. For example, when the cold-rolled steel sheet of the present invention is applied to parts for automobile outer panel, the surface quality after press molding can be secured even at the part where the surface strain was strictly controlled and the pass was difficult until now Since it can be made higher in strength than the steel plate for outer panel, it has an effect of being able to sufficiently contribute to the collision safety and weight reduction of the automobile body. Moreover, it is applicable not only as an automotive component but as a household appliance component.

Claims (3)

In mass%,
C: 0.0040 to 0.0120%,
Si: 0.70% or less,
Mn: 0.50 to 1.80%,
P: 0.005 to 0.05%,
S: 0.01% or less,
Al: 0.005 to 0.3%,
N: 0.005% or less,
Nb: 0.025 to 0.110%
Contains and
In a steel slab having a composition in which the atomic ratio of Nb to C is 0.80 or more and 1.10 or less and the balance is Fe and an unavoidable impurity represented by the following formula (1):
Finish rolling The finish rolling is performed at a temperature of 880 ° C. or higher, and wound at 550 ° C. or more and 670 ° C. or less to form a hot rolled sheet;
A cold rolling step of subjecting the hot-rolled sheet to cold rolling at a cold rolling ratio of 65% to 92% to obtain a cold rolled sheet;
And annealing the cold-rolled sheet at an annealing temperature T in a temperature range of 760 ° C. or more and 950 ° C. or less.
A steel sheet having a low yield ratio, wherein the relationship between the thickness true strain εt in the cold rolling process and the heating rate HR (° C./s) from 650 ° C. to the annealing temperature T in the annealing process satisfies the following equation (2) Manufacturing method.
(12/93) x (Nb / C) (1)
However, Nb and C in said Formula (1) represent content (mass%) of each element.
−14.0 ≦ ln (HR) −5 (εt) ≦ −8.0 (2)
However, εt in the above equation (2) is calculated by the following equation (3).
εt = −ln (1-CR / 100) (3)
However, CR in the said (3) Formula is a cold-rolling rate (%) in a cold-rolling process. In addition to the above composition, furthermore, the steel slab is, by mass%,
The manufacturing method of the steel plate with the low yield ratio of Claim 1 containing B: 0.0010% or less.   Furthermore, the manufacturing method of the steel plate which has a low yield ratio of Claim 1 or 2 which performs a plating process on the steel plate surface after the said annealing process.